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What you describe does not sound like an emergency. The pimples you describe are consistent with a bacterial skin infection or pyoderma. This does not require immediate treatment, but she will need to be seen at your regular vet on Monday.
She will need oral antibiotics or an injection of antibiotic from your vet to clear this up. In the meantime, is she itching or chewing at herself?
The pustules are not caused by allergy alone - I suspect that she has an underlying allergy. The allergy can cause inflammation of her skin, and any scratching or itching also breaks down the outer layer of the skin. This allows bacteria to get into the skin and cause a superficial infection. Basically, I think she has an allergy which led to the infection.
I think it would be a good idea to continue the Benadryl for her comfort. I am concerned about the lethargy. If you see any vomiting, diarrhea or difficulty breathing, that would warrant a visit to the emergency hospital today.
This type of infection is typically caused by the bacteria that normally live on the skin (staphylococcus). There is always a population of bacteria and yeast, but as long as your skin is healthy, your natural defenses prevent any problems. When there is a breakdown in those defenses (which can happen with allergies), that bacteria can begin to cause problems. If the swelling does not respond to Benadryl, she may need to be treated with steroids.